Without limiting the scope of the invention, its background is described first in terms of general system design and second in connection with a system which implements an analog-to digital converter function, as an example.
Heretofore, in the field of system design in general, there have been several approaches to designing extremely accurate systems and/or correcting inaccuracies which arise after the system is physically realized. Some typical approaches are: feed-back, wherein a scaled version of the output is continuously fed back and compared to the input, and the difference or error is used to correct the system or the output so that the desired output is achieved; correlated double sampling, wherein a zero or reference input is sampled between sampling of the input data stream, and the output used is calculated as the difference of the output generated by the reference and the output generated by the input data; look-up tables, which are used to adjust the parameters of the transfer function and thereby improve system performance; histogram analysis, wherein a deterministic or random input signal is applied to the system and the output is observed to determine its actual transfer function, this information is then used to drive compensating logic or functions; neural networks which implement a system by training a network using feedback with adjustable parameters to obtain a desired output response for a specific or set of specific input responses.
Some of the problems faced by the prior art approaches have been that many of the techniques listed above require measurements of the transfer function of the system with components which are more precise than the system itself, or alternatively operate to attempt to drive the components to their designed values through self-calibration techniques.
If the system is designed to attempt to control the component values, the precision possible is limited by the accuracy of the components. If the system uses digital error correction techniques, greater precision is possible but the values of the components must be physically measured. If the system is a closed system, such as an integrated circuit, measurements may be impossible or costly, or require actual fabrication of the measuring hardware- which is in fact a more accurate system; perhaps requiring accuracy beyond that available with the state of the art.
Even if the system is measured and precisely calibrated at the outset, the components will in time experience drift. The device must be periodically recalibrated, and the result is that it may be necessary for the precise measuring device to remain in place.
Other calibration approaches require continuous error correction calculations, which interfere with and limit the rate at which the devices can accept incoming data. Some approaches require precise reference signals to be available. Finally, all of the above approaches are limited to the inherent inaccuracy of the calibration components. This fundamental limit forms an obstacle to the development of highly accurate, high-speed systems. In the example of analog-to-digital or ADC circuitry, this limit appears to prevent the development of converters that are highly accurate (&gt;10 bits) and high speed (tens of Mhz) converters. Accordingly, improvements which overcome any or all of the problems are presently desirable.